Pressure-volume (p-v) relationships (or "compliance curves") obtained during colonic balloon distention are used to assess drug effects. P-v curves are also applied to characterize biomechanical properties such as viscoelasticity. This is possible if the experimental conditions are consistent with the assumptions of biophysical models. Our aims were: i) To demonstrate that the deforming stress (i.e. intraballoon pressure) and tissue tension (represented by balloon volume) are in equilibrium (or "steady state") during colonic balloon distention in humans; to assess ii) the intraindividual reproducibility of p-v relationships, iii) the effect of altering the rate of inflation on p-v curves and iv) differences between ascending (Asc) and descending (Des) p-v curves, i.e. hysteresis. Methods: A barostat-balloon was positioned in the cleansed descending colon in 15 healthy volunteers. During each p-v curve, the barostat balloon was inflated (Asc curve) and subsequently deflated (Des curve) in 4 mmHg steps from 0-44 mmHg. At each step, balloon pressure was maintained until balloon volume stabilized, i.e. fluctuation <10% over 30 seconds, subject to a maximum duration of 5 min for Pr ?24 mmHg, 2.5 min for 28-44 mmHg. Four p-v curves at different distention rates (10, 30, 30 and 60 ml/second) were performed in randomized order in each subject. Analysis: The variability in balloon volume at any given pressure was used to determine if "steady state" was achieved. For reproducibility, p-v curves at 4 distention rates were analyzed using an exponential function defined by ??and ? using the intraballoon volume during last 30 seconds at a given pressure. Results: Balloon volume reached "steady state" at ?2 minutes of beginning distention at each pressure step, i.e. variability (CV%) ?10% (at 2 min) for Pr ?16mmHg and <5% (at 1 min) for Pr ?20 mmHg. Asc and Des curves were reproducible within subjects. Asc and Des curves were significantly different indicating hysteresis. Order of distention did not significantly influence p-v curves. Coefficient of variation (CV%) between subjects denoted in parentheses? * p<0.05; ** p <0.01 (paired t test: Asc vs. Des) Conclusions: Colonic balloon volume reaches "steady-state" during assessment of p-v relationships in humans. P-v relationships are reproducible within individuals, demonstrate modest interindividual variability and the phenomenon of hysteresis.